Moon

The Moon is Earth's natural satellite and the primary celestial object that orbits our planet. It orbits Earth at an average distance of about 384,400 kilometers and completes an orbital period of about 27.3 days (a sidereal month). Because of tidal locking, the Moon shows the same face to Earth most of the time, creating a near side and a far side. The Moon's diameter is about 3,474 kilometers, its mass about 7.35 × 10^22 kilograms, and its surface gravity about 1.62 meters per second squared. Its mean density is 3.34 g/cm³. There is no significant atmosphere; instead, a tenuous exosphere exists.

Geologically, the Moon comprises a crust, mantle, and a small core. The near side features large basaltic plains called maria, formed by ancient volcanic activity, while the far side has a thicker crust and more highlands. The surface is heavily cratered and covered with regolith generated by micrometeoroid impacts.

The leading theory of origin is the giant impact hypothesis: a collision between the early Earth and a Mars-sized body about 4.5 billion years ago, with debris coalescing into the Moon. Isotopic similarities between Earth and lunar rocks support this idea.

Human and robotic exploration has expanded knowledge of the Moon. Six Apollo missions (1969–1972) landed humans and returned about 382 kilograms of samples. Robotic missions have mapped the surface and gravity and detected water ice in permanently shadowed polar craters. Modern programs, such as NASA's Artemis, aim to return humans to the Moon for sustained exploration.